Hi AJS,
When it comes to filters everyone has their own opinion. The best advise is to try to understand the reasoning behind the opinion and see if it makes sense.
For myself I would not buy either of those filters for a SW application. A Bio wheel is not recommended for SW that incorporates live rock. There is a lot of info about using artificial bio media for SW on the net but, in short, live rock is far more efficient and the bio wheel may tend to stop turning with the build up of salt creep (evaporated salt crystals). The Whisper series doesn't impress me with its disposable filter pads that will have to be cleaned frequently for SW applications. It will be an expensive filter to maintain.
If you want a HOB (hang on back) type of filter I would recommend the Aqua clear series, especially with an accessory called a surface skimmer. The sponge will last for a long time, it is easy to add carbon or phosphate remover and the filter is very easy to clean.
Another common filter I see in my travels is the Fluval cannister filter. They are easy to clean as well, can be tucked under the cabinet and are more silent than HOB's. They can also be fitted with a surface skimmer attachment.
The main point to remember when using a power filter (regardless of brand) is that they need to be cleaned frequently so a model that is quick and reliable should be considered.
Here's a link to an article I wrote showing how a power filter can be used on a small SW tank.
http://www.aquahobby.com/articles/e_small_aquarium_filter.php

Percula,
Why would you recommend a Rena over a Fluval? Providing some qualification is far more valuable to someone trying to make a decision than a simple blank statement (see my post above).
I recommend Fluvals because, as an Aquarium maintenance service provider, I work on about 15 tanks that run Fluvals as their mechanical filtration without issue. I find them reliable and easy to maintain. The models are usually 305's, 405's or FX-5's. I also run a FX-5 as the main filter on my own tropical reef._________________Intelligence is not having all the answers; it's knowing how to think!